The present invention relates to an electron microscope, and more particularly to an electron microscope equipped with a means capable of suitably preventing sample contamination.
To measure pattern widths of very fine processed products such as semiconductor devices and to investigate outer views of these processed products, electron microscopes such as scanning type electron microscopes are used. When these very fine processed products (will be referred to "samples" hereinafter) are irradiated by electron beams, sample contamination will occur. This sample contamination may be caused by such a phenomenon that gas emitted from the sample, or a very small amount of CO.sub.2 and H.sub.2 O existing in an atmosphere adjacent the sample is decomposed/recombined by receiving electron beams, and thus the gas will be converted into a carbon film and a hydrocarbon film, which are deposited on the surfaces of this sample. There is a risk that when the sample contamination caused by the electron beam irradiation occurs on, for instance, a photoresist pattern during a manufacturing stage of a semiconductor integrated circuit, this sample contamination may cause problems in the subsequent manufacturing stages such as the exposing stage and the etching stage, resulting in defective integrated circuits. Also, since the outer shape of the pattern to be microscopically observed would be varied due to this sample contamination, there is another problem that the incorrect measurement values would be acquired.
To avoid this sample contamination, otherwise to suppress adverse influences caused by the sample contamination within such a range where the manufacturing process could not be damaged, the irradiation amount of the electron beams (will be referred to as a "dose of irradiation" hereinafter) expressed by the following formula (1) must be suppressed to the minimum irradiation amount: EQU Dose of Irradiation=Irradiation current per unit area.times.Irradiation time(1)
Conventionally, in order to reduce an irradiation dose, various methods have been employed as reported in Semiconductor World 1985, 8, page 107. That is, unnecessary beam irradiation is avoided by employing the beam blanking operation, or the S/N (signal-to-noise ratio) is improved by way of the image processing operation to thereby produce the images having high image qualities under less irradiation dose. Further, in the latest electron microscope having the pattern width measuring function, the visual field selection, the image adjustment, and the length measurement are controlled by the CPU with employment of the image processing technique, so that the dose of irradiation could be minimized.
On the other hand, in the case that the microscopic image is manually adjusted by the operator, or the electron microscope is used as the outer view inspection apparatus, the irradiation dose is actually lowered in such a manner that the irradiation current is reduced while the S/N is not deteriorated. As a consequence, no specific consideration is made of the area into which the electron beams are irradiated, and also of the irradiation time.